The present invention relates generally to board coating compositions and specifically to binders for paperboard which comprise a cationic starch and a vinyl acetate polymerization product. More specifically, the invention provides binder materials in which vinyl acetate grafted starch dispersions and blends of cationic starch and polyvinyl acetate are used to replace protein typically used in such formulations.
Compositions for coating of paperboard are well known in the art and comprise a pigment component, a binder component and miscellaneous components such as lubricants, dispersants, defoamers, biocides and preservatives. Pigments useful for board coatings are similar to those useful for paper coating and typically include materials such as clays, calcium carbonate, titanium dioxide as well as colored pigments. While binder components for paper coating compositions generally comprise starch and/or latex, board coating compositions typically require greater pigment binding strength than can be provided by starch and/or latex alone. Accordingly, board coating binder compositions additionally comprise protein, such as soy protein or casein, as a binder, to improve binding strength sufficiently for the coated board to be useful in the manufacture of boxes, cartons and other items. The incorporation of protein into the board coating binder compositions provides sufficient pigment binding strength to those compositions to improve the strength properties of the coating as measured by glueability tests which measure the ability of one coated substrate to adhere to another coated or uncoated substrate. Unfortunately, protein is relatively costly and is difficult to handle since it requires a substantial cooking step before it can be used in the coating. Accordingly, there is a desire in the art to replace protein with a less costly binder component which does not have the protein cooking requirement.
Of interest to the present application is the observation that cationic starch is generally undesirable for use as a binder component in a paperboard coating composition because of the tendency of the cationic starch to floc clay.
Japan patent Sho 58 (1983)-185640 and Sho 59 (1984)-30827 disclose redispersible powders prepared by adding cationic compounds including cationic starch to nonionic or weakly ionic vinyl-type aqueous polymer emulsions or prepared by emulsion-polymerization of vinyl monomers including vinyl acetate in a system where cationic group containing compounds such as cationic starch or cationic polyvinyl alcohol are introduced.
Japan patent Sho 55 (1980)-12867 discloses a process of producing vinyl alcohol grafted starch utilizing the oxidized granular, tapioca and acid treated starches as the starting materials. Vinyl acetate was grafted to these starch granules at 40.degree. C. using a variety of initiators such as ceric salts, ammonium persulfate and hydrogen peroxide. The vinyl acetate grafted starches were saponified with sodium hydroxide and sodium hypochlorite. The saponified products were used in paper surface sizing applications.
Buikema et al., U.S. Pat. No. 4,146,515 discloses the use of cationic starch as a paper sizing material.
Kronfeld, U.S. Pat. No. 3,052,561 discloses the use of cationic starch derivatives as a partial or complete replacement for starch used in a paper coating binder. The patent further discloses that the cationic starches may be used in conjunction with (or in complete replacement of) other binders such as synthetic resins or latices.
Powers et al., U.S. Pat. No. 3,598,623 disclose carboxyl starch amine ethers wherein the anionic substituent to amine-ether substituent molar ratio is at least 1:1 and is preferably between about 1.1:1 and about 3:1. The starch amine has an average of from about 0.75 to about 5 amine ether substituents per 100 anhydro-glucose units which is a cationic degree of substitution (D.S.) of from about 0.0075 to about 0.05. The net negatively charged amphoteric carboxyl starch amine ethers are disclosed to be suitable for use as the sole binder ingredient in coating color compositions or for use in combination with one or more other binder materials such as a starch casein, polyvinyl alcohol, or a synthetic polymer latex including styrene-butadiene copolymer, ethylene-vinyl acetate copolymer, acrylate copolymer and vinyl acetate polymer.
Cescato, U.S. Pat. Nos. 3,654,263 and 3,706,584 disclose a cationic starch derivative with a cationic degree of substitution of from about 0.1 to about 0.01 which is oxidized to introduce carboxyl groups in sufficient quantity that the ratio of the percentage of the carboxyl groups by weight, dry basis, to the degree of substitution with the cationic-type substituent is at least 1 to 1. The oxidized cationic starch derivatives are disclosed to be useful as binders in paper coating color compositions and a stated object of the patents is to provide properties that approach those of casein for paper coating applications.
Rutenberg et al., Starch Derivatives: Production and Uses, pp. 355-388, disclose at page 363, the use of cationic starch as a paper coating binder offering increased strength due to electrochemical binding of clay to fiber. The publication notes that if the amino content is too high, agglomeration or coagulation of the clay can occur.
Mazzarella et al., TAPPI, Vol. 49, pp. 526-532 (1966), disclose the use of cationic starches as substitutes for casein in paper coating binder systems. Synthetic latices including polyvinyl acetate, styrene-butadiene and acrylic latices are suggested for use as cobinders with the cationic starch but no suggestion is made that a cationic starch/vinyl acetate product would have properties suitable for use as a board coating binder.